It is now well known to utilize a piece of low coercive force, high permeability magnetic material as an EAS marker. Such markers were perhaps first disclosed in the French Pat. No. 763,681, issued in 1934 to Pierre Arthur Picard. More recently, it has become relatively well known to use particularly configured pieces, such as elongated strips of high permeability material, in order to enhance the production of very high order harmonics, thereby improving the reliability with which such markers can be distinguished over signals from other articles such as briefcase frames, umbrellas, etc. Such uses are exemplarily set forth in U.S. Pat. Nos. 3,665,449, 3,790,945 and 3,747,086. As such elongated strips are generally detectable only when the interrogating field is aligned with the strips, it is also known from such disclosures to provide for multi-directional response by providing multi-directional fields in the interrogation zone and by providing additional strips in an L, T or X configuration. Alternatively, in U.S. Pat. No. 4,074,249 (Minasy), it is proposed that multi-directional response may be obtained by making the strip crescent-shaped. Furthermore, it is known from U.S. Pat. No. 4,249,167 (Purington et al.) to make a deactivatable multi-directionally responsive marker by providing two elongated strips of permalloy arranged in an X configuration with a few hard magnetic pieces adjacent and co-linear to each of the permalloy strips.
While still recognizing that an elongated, or "open-strip" configuration is desired in order to obtain a very high order harmonic response, U.S. Pat. No. 4,075,618 (Montean) discloses that a marker capable of generating very high order harmonics, thereby being operative in a system such as described in the '449 patent, could be made by adding flux collectors to a short strip of high permeability material which is insufficiently long to meet the definition of an "open-strip". Picard also suggests that polar extensions may be provided to increase the sensitivity, while Fearon '945 suggest the use of pole piece coupons to collect flux.
Markers such as disclosed by Elder, Fearon, Peterson, Minasy and Montean in the above patents have all enjoyed certain commercial success. However, the use of the markers has been restricted by the size, and still primarily elongated shape heretofore believed to be necessary.
EAS systems in which the markers of the present invention are particularly useful typically produce within the interrogation zone fields in a variety of directions. For example, as disclosed in U.S. Pat. No. 4,300,183 (Richardson), such differently directed fields may be produced by providing currents in coils on opposite sides of the interrogation zone which are alternately in-phase and out-of-phase. The resulting aiding and opposing fields at any given location may be appreciably weaker in one direction than another. Accordingly, a given marker may be unacceptable if reliably detectable only when oriented in the direction associated with the strongest fields produced by the EAS system. Preferably, a commercially viable marker would have sensitivity so as to be reliably detectable regardless of how it is oriented in the zone, however, in a practical sense, it is not necessary to detect markers in each and every orientation and/or location in the zone.
Typical EAS systems designed to be used with elongated "open-strip" type markers are the Model WH-1000 and 1200 systems marketed by Minnesota Mining and Manufacturing Company. For example, such systems typically produce within the interrogation zones magnetic fields alternating at 10 kHz, and having minimum intensities at the center of the zone of approximately 1.2 oersteds (Oe) when the fields generated in coils on opposite sides of the zone are in an opposing configuration and of approximately 2.4 Oe when in an aiding configuration. The receiver portions of such systems process signals from receiver coils positioned within panels adjacent to the interrogation zone, and activate an alarm circuit in the event signals corresponding to very high order harmonics of the applied field are detected.
To compare the performance of various markers, it is convenient to use a test apparatus which generates fields alternating at a predetermined frequency and has controllable strength comparable to those encountered in such EAS systems. The test apparatus should detect signals in accordance with the harmonic characteristics relied upon in such systems and provide sensitivity values, based on a standard marker to ensure valid comparative results.
Such a test apparatus is preferably calibrated against a present commercially available marker such as type WH-0117 Whispertape brand detection strip manufactured by Minnesota Mining and Manufacturing Company, which is formed of an amorphous metal 6.7 cm long, 1.6 mm wide and 0.02 mm thick and having the following nominal composition (at %): Co:69%; Fe:4.1%; Ni:3.4%; Mo:1.5%; Si:10%; and B:12%, and which is available from Allied-Signal Corporation as type 2705M. Such a marker is inserted parallel with the field of the test apparatus and the gain is adjusted to indicate a standardized sensitivity value of 1.0 at a 10 kHz field of 1.2 oersteds, that being the minimum field strength at which such a marker would be expected to be reliably detected. At a higher field of 1.4 oersteds, a sensitivity of 4.8 was observed when the amorphous marker was similarly aligned.
It has long been desired to minimize the length of such elongated markers. However, short strips do not have sufficient sensitivity to be even marginally acceptable even at a high field strength and even when dimensioned to maximize high order harmonic response. Similarly, when short pieces are further dimensioned with polar extensions proportional to that depicted in FIG. 7 of Picard, in which the length of the center section is about eight times the center width and the overall length about 13 times the center width, the sensitivity is still unacceptable. For example, a 0.02 mm thick ribbon of the amorphous metal described above was cut to provide 2.5 cm long strips 1.6 mm, 0.8 mm and 0.5 mm wide. Also, a 2.5 cm long piece, 1.6 mm wide was provided with polar extensions according to "Picard". Relative sensitivities shown in the following table were then determined using the same procedure described above.
______________________________________ "Picard" marker with polar extensions (Oe)StrengthField on each end of a 1.6 mm wide strip ##STR1## ______________________________________ 1.2 0.02 0.014 0.034 0.037 2.4 0.26 0.18 0.18 0.017 3.0 0.46 0.28 0.25 0.025 ______________________________________
It may thus be recognized that regardless of whether the strips were made very narrow, thus minimizing the demagnetization effects, or were made wider, thus providing a greater total mass, in all cases an unacceptable sensitivity level resulted. While the standardized sensitivity values of 0.02, 0.26 and 0.46 observed at the three field strengths noted above for the "Picard" type marker were superior to that observed for a strip alone, showing that increases in sensitivity do result by adding polar extensions as taught by the prior art, such benefits are still not sufficient to result in even a marginally acceptable marker.